Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, larger only than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmosphere and has a crust primarily composed of elements similar to Earth's crust, as well as a core made of iron and nickel. Mars has surface features such as impact craters, valleys, dunes, and polar ice caps. Mars has two small, irregularly shaped moons, Phobos and Deimos.
Some of the most notable surface features on Mars include Olympus Mons, the largest volcano and highest-known mountain in the Solar System, and Valles Marineris, one of the largest canyons in the Solar System. The Borealis basin in the Northern Hemisphere covers approximately 40% of the planet and may be a large impact feature.[21] Days and seasons on Mars are comparable to those of Earth, as the planets have a similar rotation period and tilt of the rotational axis relative to the ecliptic plane. Liquid water on the surface of Mars cannot exist due to low atmospheric pressure, which is less than 1% of the atmospheric pressure on Earth.[22][23] Both of Mars's polar ice caps appear to be made largely of water.[24][25] In the distant past, Mars was likely wetter, and thus possibly more suited for life. It is not known whether life has ever existed on Mars.
Mars has been explored by several uncrewed spacecraft, beginning with Mariner 4 in 1965. NASA's Viking 1 lander transmitted the first images from the Martian surface in 1976. Two countries have successfully deployed rovers on Mars, the United States first doing so with Sojourner in 1997 and China with Zhurong in 2021.[26] There are also planned future missions to Mars, such as a NASA-ESA Mars Sample Return set to happen in 2026, and the Rosalind Franklin rover mission, which was intended to launch in 2018 but was delayed to 2024 at the earliest, with a more likely launch date at 2028.
Mars can be viewed from Earth with the naked eye, as can its reddish coloring. This appearance, due to the iron oxide prevalent on its surface, has led to Mars often being called the Red Planet.[27][28] It is among the brightest objects in Earth's sky, with an apparent magnitude that reaches −2.94, comparable to that of Jupiter and surpassed only by Venus, the Moon and the Sun.[16] Mars has been observed since ancient times. Over the millennia has been featured in culture and the arts in ways that have reflected humanity's growing knowledge of it.
The history of observations of Mars is marked by the oppositions of Mars when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars, which are distinguished because Mars is close to perihelion, making it even closer to Earth.[29]